The invention resides in a method for operating and controlling hysteresis motors, which are energized with a rotary voltage from a three-phase alternating current supply or a frequency converter and which are individually monitored by an operating monitoring system and which are controlled by changing the three-phase voltage depending on this monitored values.
The design principle of such hysteresis motors with permanent magnet poles impressed into the armature is shown and described in DE 32 32 914 C1. A method for optimizing the power input of a plurality of hysteresis motors arranged in parallel is known from DE 37 57 904 A1. In that case, the hysteresis motors are all energized by a common voltage source with a rotating voltage whose voltage level is adjustable. When, at nominal voltage, the motors have reached synchronous speed and the motors have switched from an asynchronous to a synchronous operation, the voltage is reduced while the synchronous motor speed is maintained until an optimal operating point with minimal effective and idle power consumption has been reached. This way of operating the motors however has certain problems since the operating range reserve, which is needed for a safe operation, becomes smaller the lower the rotating voltage becomes. The process therefore did not persist in practice. The efficiency of the hysteresis motors obtained remains therefore substantially below the theoretically possible values, since a substantial voltage reduction is not acceptable when taking the safety aspects referred to above into consideration.
It is the object of the present invention to provide a method of operating such motors in such an improved way that the efficiency of hysteresis motors is improved to over 90%, while operational safety and the capability of managing disturbances are maintained.
In a method of operating and controlling hysteresis motors energized by a three-phase power supply and individually monitored by an operating monitoring system, whereby the effective power output of the individual motors is determined and the three-phase voltage is changed if the effective power output deviates from a predetermined range, the hysteresis motors are first energized at an intermediate range voltage until they reach nominal speed and are running at synchronous speed, the supply voltage is then increased to an upper range and then reduced to a lower range for continuous operation of the motors, and the power output of the individual motors is monitored and, if it drops below a threshold indicating a switch to asynchronous operation, the operating voltage is again increased to the intermediate range for renewed synchronization.
By impressing the permanent magnets into the armature, which consists of a magnetizable steel, at a voltage, which is greater than two and half times the operating voltage, at nominal voltage, an increased efficiency with a flat maximum in the lower voltage range is achieved, when the rotating voltage decreases. This maximum is utilized with the present invention for continuous operation of the motors while maintaining a safety reserve. In contrast to the state of the art, in this lower voltage range not the increase of the effective power output is monitored but the shifting of a particular motor to asynchronism. As soon as this occurs, the voltage is increased to the intermediate range and the motor is again synchronized at normal frequency. By way of the monitoring system, it can be determined whether the shift to asynchronism is caused by a passing disturbance or the disturbance is a permanent. In the latter case, it is necessary to intervene from without.